GREY BOX PREAMP

This is a clean preamp that I designed to use with my bass, a power amp, and a cab at a rehearsal space. (Rehearsal space being a fancy name for the singers basement.)

Design Goals

• Flat frequency response.
• 20dB gain.
• 2V p-p input.
• No controls.
• Tuner out.
• Handle output to low input impedance.
• Run off cheap wall wart.

Schematic

Design

I am very proud of this preamp since, unlike the mu-preamp, I designed it from the ground up. Although there is nothing special about the design, at least it is all mine.

The design started as an all transistor design using only MPSA18s. (Original design) The original design had three stages: input buffer, first gain stage, second gain stage. I did not design the input buffer, I got it from the Acoustic Control 360 Bass Preamp schematic.

I breadboarded the original design and it worked. Then I realized that a FET could provide both the high input impedance and the first gain stage. So I switched to the current design. And got rid of the part I did not design in the process.

You may notice that the every stage has paired transistors. The second transistor acts as a buffer. For the first stage, it provides a high impedance to the FET to maximize its gain.

For the second stage, the buffer is required to provide a nice low output impedance for the power amp. The power amp I am using has a 10K input impedance. I measured the output impedance of the NPN gain stage without the buffer at about 3.5K. With the buffer it is about 16 ohms. Much better.

This design has a much cleaner sound than the mu-preamp. However, guitar players might like the sound of the mu-preamp better.

Design Notes

• The preamp is designed to handle a maximum input of 2V p-p without clipping at max gain.
  
  
• The first stage is designed for a gain of 2.5 and an Ids of about 1mA. The second stage is designed for a gain of 4 and an Ic of 5mA. This provides a gain of 10 which is 20dB.
  
  
• I used a 2N5458 for the FET. Other FETs could be used. I would not recommend the common J201 since it would be running at it's maximum Idss.
  
  
• I used MPSA18s for the NPN transistors. It was designed for 2N5088s, so any NPNs with a minimum hfe of 350 or higher should work fine.
  
  
• I chose a 24V supply voltage because that is the largest wall wart style supply that is easy to get. The 24V supply I used produces close to 32V at the low current used by the preamp. I used an LM7824 to regulate the voltage to 24V.
  
  24V is also the about the minimum to handle 20V p-p with a 2V p-p input.
  
  
• Spice simulations show response as flat (< -1dB) from 20Hz to 20kHz.
  
  
• The current design does not have a tuner out. I have left room to put one in.
  
  
• I used to show a level control. I gave up on it since it was always scratchy. I have instead dropped a 100k pot on the output as a volume control.
  
  
• If you need a bit more gain, replace R3 with a 2.2k resistor. Note this will decrease the maximum input some.
  
  

Credits

Credit goes to the guys at the DIYstompboxes.com forum.

Extra credit goes to R.G. and George Giblet for going above and beyond the call of duty answering my questions. I owe you a beer.

Changes

April 2007 Removed gain control.

October 2006 Updated the schematic to move the gain control.

April 2006 New schematic with components given a reference. Added 1M level idea. Added decreasing R3 for more gain.

Back to the stompboxes.

Contact Sean MacLennan
Last modified: Sun Feb 14 23:10:43 EST 2010